Variations of the canalis hypoglossi.

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AUTHOR'S A B E T R A m OF rHIS P A P E R lELlUED
BY T E E B I H L I O G R A P E I C S E R V I C E A U Q C S T 11.
VARIATIONS OF THE CANALIS HYPOGLOSSI
RALPH D. LILLIE
F r o m the Division of A n a t o m y of the Stanford Medical School
My attention was attracted to the subject of the variations
of the hypoglossal canal by the discovery of a double hypoglossal
canal in the dissecting room. Subsequent examination of about
thirty skulls, about twenty of them European, revealed ten cases
of complete division of the canal. Consultation of the ordinary
reference books failed to give definite figures. So Dr. Meyer
placed the collection of skulls in the anatomical museum and
Indian skulls in the general museum at my disposal.
Bartels ('04) found the hypoglossal canal doubled in 117
cases in 958 half skulls or in 12.2 per cent. In the seventeen
varieties of skulls used by him the percentage varied from 0.0
per cent up to 27.9 per cent in Inca skulls. Weigner ('11) who
examined 103 skulls in a study of the variations of the occipital
bone found that the following percentages held for German
skulls :
Left side
Clear, 62.1 per cent
Incomplete or indicated division, 10.4
Complete division, 18.5 per cent
Right side
Clear, 59.2 per cent
Indicated division, 18.5 per cent
Complete division, 22.3 per cent
Division into three parts was indicated in several cases.
When the canal was double it was frequently divided into unequal
parts so that the posterior canal was about twice the size of the
anterior, or more rarely in the reverse ratio. Weigner believes
that the division of the canalis hypoglossi is one of the manifestations of an occipital vertebra. Graf v. Spee ('96) notes the
division of the canal and says that indications of division are
almost constant in the adult but makes no mention of the character of the indications. Jaboulay and Lucy ('11) attribute the
division of the canal to that of the nerve and Froriep ('11) showed
131
T H E ANATOMICAL RECORD, VOL.
AUGUST,
1917
13,
NO.
3
132
RALPH D. LILLIE
that in the bovine embryo the hypoglossal nerve corresponds to
three segmeatal nerves. Schafer and Symington in Quain’s
Anatomy 111, part 2, 1909, also regard the hypoglossal nerve
as representing three segmental nerves. Prentiss (’10) working on pig embryos, found eight ganglionic masses corresponding
to four ganglia associated with the hypoglossal nerves as their
dorsal root ganglia. These ganglia were in series between those
of the vagus and of the first cervical nerves. They are interpreted as representing four hypoglossal nerves. Five or six
ventral roots of the hypoglossal exist, but according to Bremer
(’08) the more anterior of these represent ventral roots of the
vagus and glossopharyngeal nerves. Martin (’91) working on
cats, found five ganglia and five ventral roots and concluded all
were hypoglossal. So it would seem that either three or four
nerves exist developmentally and this could account for either
three or four somites and their corresponding neural foramina
and their corresponding vertebrae. Hence between tlie jugular foramen and the synchondroses petro-occipitalis and occipitosphenoidalis on one hand, and the atlas on the other, there must
have existed four or five occipital vertebrae with three or four
neural foramina or hypoglossal canals between them. Consequently we should expect to find traces of division of the hypoglossal canal into either three or four parts.
In fact we do find such divisions. As stated above Weigner
noted indications of divisions into three parts, also into two
parts, the posterior of which was twice the size of the anterior.
I also have seen a number of such cases. Such occurrences could
be interpreted as partial or complete separation of the first
hypoglossal nerve while the two following remain together. I
have seen indications of division into three equal parts two of
which may be completely separated. However, complete division into three parts has not been observed.
McMurrich (’05) says “during the cartilaginous stage of the
skull the anterior condyloid foramina are divided into three portions by two cartilaginous partitions which separate the three
roots of the hypoglossal nerve,” and considers this as evidence
of the existence of four fused vertebrae in the occipital bone.
VARIATIONS O F THE CANALIS HYPOGLOSS1
133
These facts point toward three original hypoglossal canals.
But a number of cases were observed in which the canal was
completely or incompletely divided into two equal parts and in
which the anterior canal showed indications of further division
into two equal parts. In the light of the work of Prentiss and
Bremer I must look upon such instances as an indication of division of the canal into four equal parts. Of course, it may be
in this case that it is really divided into three and that the
third hypoglossal nerve is larger than the others. Pokier et
Charpy report cases in which the canal was divided into four
parts. Professor Meyer suggested to me that some of the osseous
processes observed may be secondary ossification in the dural
septa extending between the fasciculi of the roots instead of
remnants of arches of occipital vertebrae. Such a case was
noted in No. 42, a dissecting-room specimen. In this skull the
dried remains of a dural or connective tissue septum extended
between two spurs on opposite sides of the foramen. Whether
these spurs are to be looked upon as secondary ossifications of a
dural septum or whether such septa are to be considered as unossified remnants of vertebral arches can not be decided in
dried skulls.
It is also probable, indeed almost certain, that in some cases
these divisions are significant not of division of the nerve, but,
of vascular variation. Some of the variations recorded here are
unquestionably due to aberrant vessels and not to division of
the nerve. I refer here to those cases in which tortuous canals
leave the main canal more or less obliquely to reenter in another
place in similar fashion. On the other hand a large number of
these variations are in all probability due to division of the
nerve, for the nerve has been observed to pass through as two
separate nerves, each in a canal of its own. In the more caudal
parts of the body the metamerism of nerves and vertebrae holds
strictly. Metamerism of the nerves exists in the head, showing
best in the embryo. Indications of bony segmentation of the
canal corresponding to the nervous segmentation are also found.
Moreover an occipital vertebra may be partially separated.
These facts all point in the same direction.
134
RALPH D. LILLIE
OBSERVATIONS
35 Indian skulls from Jersey County, Il1inoi.s
Character of hypoglossal canals
NUMBEE
LEFT
RIGHT
7615
A slight superior spur not
at either opening
7613
7612
7630
Simple
Simple, smaller
A slight superior prominence near t h e margin
Simple, round
A septum a few millimeters thick a t internal opcning. The foramina equal
Simple
Simple, larger
A slight superior tubercle a t theinner margin
__
7609
7607
7605
7608
7606
7604
7616
7618
7619
7620
7627
7626
76'5
-_
A small antero-superior
spur
Simple
A heart shaped internal
opening wit,h a prominence superior
A small antero-superior
notch
The internal margin is
rough superiorly
h sept,um 3 mrn. wide by
I mm. thick internally.
The anterior canal is
smaller than the posterior
A slight superior spur
Opposed spurs above and
below
A broad septum within
canal. Anterior canal
half t h e size of t h e
posterior
Simple
A slight superior tuhercle
Bimplc
An inferior spinc, a flattened internal opening and a slight indication abovc a t the
internal margin. Round externally
A small antero-superior spur
A rough superior internal margin
An anterior foramen right through t o a
common external opening. A larger posterior foramen giving rise t o a small anterior canal through to the common extcrnal opening and a large posterior blind
pit. I n t h e external opening are three
small pits abovc and posterior t o t h e two
canals
A small antcro-superior notch
A marked internal superior spine with a n
indication opposite on the lower margin.
The anterior division is half the size of
the posterior
A projcction on t h e superior margin
Two equal canals
Compresscd dorso-ventrally
d broad scptum within the canal. The
anterior canal half the size of the posterior
Simple
il slight superior spur
Simple
VARIATIONS O F THE CANALIS HYPOGLOSSI
OBSERVATIONS-Continued
NUM-
LEFT
BER
-
Simple
7623
Simple
7622
Simple
7639
A superficial internal septum. Small anterior,
large posterior canal
7638
A broad superficial internal septum. Anterior
canal smaller
Slight antero-superior
spur on roof
Simple
Simple, larger
Anterior superior spur.
One t o two division
Antero-superior notch
Simple
Simple
Simple
A ridge below, a spur
above
Simple
Simple
A marked superior spur
Simple
7635
7634
7633
7631
7628
7632
7610
7624
7641
7640
7643
7642
-
RIGHT
A deep septum half the length of t h e canal.
7624
7637
135
Equal division.
A slight double spur on the internal superior margin
A slight spur from the roof near the internal orifice
A rough superior margin internally. The
medial side of the posterior condyloid
canal opens into the skull posterior t o
canalis hypoglossi
A slight superior spur. The anterior division half the posterior
Slight antero-superior notch
Simple
Simple, smaller
Slight double superior spur
Two slight superior elevations
Simple
Simple
Simple
Simple
Simple
Simple
Simple
Simple
Two I n d i a n skulls from L a Conner, Washington
7645
7644
An internal septum. Anterior canal half the
posterior
Simple
Simple
Simple
Three I n d i a n skulls from the Longueville graves, Plumas County, California
I A superior spur
IA
7648
A broad superficial inter-
narrow,
septum
Simple
7649
nal septum
Simple
Simple
7647
median,
superficial internal
RALPH D. LILLIE
OBSERVATIONS-Continued
16 I n d i a n skulls .from Ponce mound near M a y f e l d , California
NUXBER
E.
7650
7651
7652
7653
7654
7655
7657
7656
7660
7658
7662
7661
MI
hi12
__
LEFT
Simple
A thin, narrow septum
in the mid-portion of
t h e canal
Clear
Spur on the sup.erior internal margin
Simple
Simple
Simple (young, first permanent molar)
Simple
Simple
Simple, flattened
Simple
A deep internal septum
in inner third of canal
Simple
A thin, superficial internal septum. Anterior
foramen half the size of
the posterior
Both foramina round.
Septum deep i n middle
third of canal. Anterior canal larger
RIGHT
Simple
Superior tongue-like process and a slight
inferior tubercle
Complete septum. Anterior canal half the
size of the posterior
Opposed spurs on internal margin
Simple
A superior anterior roughening
Simple
Simple
Simple
Simple, flattened
Simple
Simple
Simple
Simple
Flattened from t o p down.
Five I n d i a n slculls from near S a n Jose, California
M3
hIedian septum near internal foramen
M4
Antero-superior spur and
notch
Simple, round
M5
M6
117
Broad flat septum. Two
internal foramina are
equal. External foramen single
Simple
Median septum near internal foramen.
Opposed spurs divide anterior canal into
two
Simple
Opposed spines. Anterior division half
posterior
Basioccipital of this side is destroyed
Strong superior and slight inferior spines
on internal margin. Anterior division
half posterior
VARIATIONS OF THE CANALIS HYPOGLOSSI
137
Twelve whole skulls or entire occipital bones
NUMBER
LEFT
RIQHT
Completely divided nerve Clear. Only one hole i n dura
1
in both canals
Rough superior internal Rough superior internal margin
2
margin
Slight median superior Clear. (Anterior part atlas assimilated)
3
spur
Clear
4
(102.) Clear
Clear
Antero-superior notch
5
Medium superior spur.
Opposed median spurs
6
Also superior spur i n
middle of anterior division
Clear
7 Clear
(13.) Strong septum an- Septum in middle third of canal. Ante8
terior canal 2-3 times
rior canal is twice the size of the posterior
as large a s posterior
An anterior-superior notch Anterio-superior notch. (Occipital only)
9
Clear. (Occipital only)
Clear internally.
At external opening a
10
small superior canal cut off
Clear
Clear
11
Slight median superior C1ea.r
12
elevation. Two small
orifices just above internal opening
~~
-
Twenty-two European skulls
IV. Clear
Antero-superior cleft and spur
1
2
V. Thin median septum Thicker median septum i n middle third.
i n inner third
Spur in middle of Vuperior margin.
Small inferior spur and opposed superior
ridge i n middle of anterior canal
XIII. Large and clear, Large and clear
3
posterior condyloid canal opens laterally
Median superior tubercle
11. Clear
4
XV. Antero-superior ir- Antero-superior and antero-medial spurs.
5
Small canal antero-superior t o two spurs
regular process 1 t o 2
mm. long
Thin septum in middle Opposed elevations. Anterior part con6
third of canal. Antestricted and half the size of the posterior
rior two-thirds size of
posterior. ’ (6 on medial
side of left mastoid)
RALPH D. LILLIE
OESERVATIONS-Continued
NUMBER
RIGHT
-
16
17
XII. Opposed antcro-superior and -mrdial spurs
which enclosc a sinall
canal
XIV. Clcar
VII. A n t e r o -sii p e r i o r
marginal elevation
111. Slight antcro-superior tubercle
XVII. Triangular.
A
fossa runs hack from
posterior superior corner into a small pit
IX. Clear opening. A
small venous canal runs
antero-mcdlally
from
roof ncar posterior orifice t o ncar the external
orifice of canal
XVI. Clear
VIII. Clear
Heavy septum. Anterior
canal small, almost slitlike. The slit points
toward the condyle
V. Clear
111. Clear
18
11. Strong septum in in-
7
8
9
10
11
12
13
14
15
19
20
21
22
Same a s left but cnclosed canal smaller
Clear
Clear
Clear
Antero-superior notch clear through
Clear
Clear
Clear
Clear
Clear
A long superior spine which almost divides
the main canal into a smaller anterior
canal and a larger posterior. A wide
opening in the postero-lateral wall opening into the posterior condyloid canal.
Clear
ncr half. Anterior canal
slight.ly smaller
(04.) (Separate occipital) Median superior spur
Clear
B11. Opposed median Clear
spurs, t h e superior being stronger
Clear
C11. Clear
Clear
Clear
139
VARIATIONS O F THE CANALIS HYPOGLOSSI
_
_
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n
W
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SIDE
LOCALITY
4
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Illinois Indians. . . . . . .
California Indians.. . . ,
Specimens from dissecting room .... . . .
European skulls.. . . . . .
Ew
$
8
4
Left
Right
Both
n
14
17 48.6
15
15 42.9
29
32 45.7
I
Left
Right
Both
14
16
30
56.0
66.7
61.2
I
Left
Right
Both
31
31
62
43.7
43.7
43.7
30
31
61
Left
Right
Both
13
13
26
Left
Right
Both
75
75
150
~~
1
I
I
1
Total.. . . . . . . , . . . . . . . .
1
Ew Em
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E2
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40.0
42.9
41.4
4
5
9
11.4
14.3
12.9
35
35
70
3 12.0
5 20.8
8 16.3
8
3
11
32.0
12.5
22.4
25
24
49
42.2
43.7
42.9
10
9
19
14.1
12.7
13.4
71
71
142
59.1
59.1
59.1
5 22.7
8 36.4
13 29.5
4
1
5
18.2
4.5
11.4
22
22
44
49.0
49.3
49.1
52 33.9!
59 38.8:
111. 36.3!
26
18
44
16.99
11.84
14.43
153
152
305
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L'npaired series.
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Fifty-nine left sides of skulls or of occipital bones f r o m cadavers
dissected in the dissecting room
1. Anterior superior spine and notch.
2. Anterior superior spine and notch.
3. Clear. A small canal from the lateral wall of the canal opens behind t h e
ocripital condyl. Anterior t o this are two other apparently blind small canals.
4. A superior tubercle in the middle of the internal margin and another
midway between t h e first and the antero-medial internal margin.
5. Clear.
6. A strong septum with a superior spur projecting inward. The anterior
canal is smaller.
7. The superior wall of the canal is destroyed, the remainder appears clear.
8. A strong superior spur internally. The anterior division i s half the
posterior.
9. Clear.
10. A small superior elevation between anterior and middle thirds of the
internal margin.
11. Clear.
12. Large. Rough superior and antero-medial margin.
13. (13L.) A strong complete septum clear through t o the external opening. The antcrior canal is half the posterior. A canal from the postero-lateral
140
RALPH D. LILLIE
wall of the posterior canal opens into the medial side of the posterior condyloid
canal. Anterior canal has marked anterior construction. From anterior opening the anterior canal has marked superior diverticulum which communicates
with top of posterior canal. Latter has smaller anterior opening.
14. Clear.
15. Large, clear.
16. Posterior condyloid canal opens just posterior t o posterior internal margin. The internal orifice had slight antero-superior notch. Opposed superior
and inferior ridges near the external orifice divide t h e canal evenly.
17. Antero-superior cleft.
18. Clear. One pit i n superior, another i n postero-lateral wall of t h e canal.
19. Rough superior margin. A small vascular canal goes laterally then
forward from the posterior internal margin and reenters the canal a t the external
orifice.
20. Afitero-superior cleft internally.
21. The superior wall is destroyed. Rest clear.
22. Strong median superior spine internally.
23. Small antero-superior cleft.
24. Clear.
25. Clear.
26. Clear.
27. (27L.) A strong internal septum divides t h e canal equally. The internal orificc of the posterior canal is much larger. A wide postero-lateral opening into the posterior rondyloid canal and a large superior diverticnlum opening
posteriorly and internally. The internal opening of the anterior canal has a
sharp spine from the antero-medial wall with a deep cleft below. A superior
diverticulum from the conimon external opening.
28. Clear.
29. Clear Slight antero-medial constriction.
30. A septum in the outer t,hird or half of t h e canal. The posterior canal is
half the anterior.
31. Antero-medial or -superior notch.
32. Large. Antero-medial notch. Posterior condyloid canal has opening
into postero-lateral wall.
33. Opposed spurs. Anterior division is half or third the size of t h e posterior.
34. Antero-superior spur.
35. (35L.) Internally a n antero-superior spur has bridged t h e canal completely, cut,ting off a small sntero-superior canal which has a median superior
spur t h a t makes it U-shaped.
36. Rough superior margin. Anterior constriction. Two small canals from
postero-superior wall probably t o the posterior condyloid canal.
37. Clear.
38. Clear.
39. Antero-superior notch. Canal from postero-superior internal margin
probably t o posterior condyloid canal.
40. Clear.
41. Median and anterior superior notches.
42. Clear.
VARIATIONS OF THE CANALIS HYPOGLOSSI
141
43. Clear.
44. Clear.
45. Internal superior marginal spur, between middle and posterior thirds.
46. Clear.
47. Antero-superior spur.
48. Antero-medial tubercle.
49. A superior spine and a n opposed inferior elevation. Almost complete
division, so t h a t the posterior division is one and half times the size of the
anterior division.
50. Antero-superior internal notch includes a small spine and opposed spines
i n the middle of the canal dividing i t equally into postero-superior and anteroinferior parts.
51. Antero-superior notch.
52. Opposed spurs. Anterior canal smaller.
53. Clear.
54 Median superior notch.
55. Strong median septum.
56. Opposed spines-superior longer. Anterior c'anal smaller.
57. Clear.
58. Clear. Only one hole in dura mater.
59. Large. Strong slightly anterior septum in middle third of canal.
Fifty-nine right hnlves
I. Antero-superior cleft.
2. Thin septum which is incomplete a t the inner opening. Anterior canal
smaller with further anterior notch.
3. Flattened from above down. An indicated spur in the middle of the superior internal margin. Anterior half of orifice especially flattened. External
orifice rounded.
4. Superior median elevation and a n anterior superior spur with a n opposed
inferior elevation.
5. Narrow septum. Anterior canal smaller.
6. Clear.
7. Clear.
8. Rough superior margin. Elevation between anterior and middle thirds
of superior internal margin.
9. Antero-superior notch.
10. Constricted anteriorly.
11. Strong septum with superior internal spur. Equal canals.
12. Clear.
13. Small antero-superior cleft.
14. Opposed spurs which separate the anterior third of the canal. I n addition there is a n antero-medial spur.
15. Clear.
16. Greatly flattened. Median spur within the canal on its roof.
17. Clear. Flattened.
142
RALPH D. LILLIE
18. (I8R.) Large oval inner orifice. At the outer orifice a thin strand of
bone divides off a smaller posterior division. Postero-lateral t o this is a pocket
opening back and inward with a round hole 1 mm. i n diameter i n the middle which
opens through into the external orifice and a pit 2 m r n . in diameter above, which
opens anteriorly into a large superior diverticulum from the anterior canal.
19. Median superior spine.
20. Clear. Large canal from posterior wall to posterior condyloid canal.
21. Clear.
22. Clear.
23. Antero-supcrior spur and notch.
24. Strong septum. Anterior canal smaller. Common external opening has
superior diverticulum.
25. Slight spur internally on upper third of antero-medial margin.
26. Clear.
27. Superior spur on inner margin posterior division half the size of t h e
ant,erior.
28. Clear.
29. Antero-superior spur and notch.
30. Clear. Foramen magnum and condyle malformed.
31. Clear.
32. 1,argc internal opening clear. A canal starts off from the middle of the
postero-superior wall opening separately a t outer opening. An antcro-medial
notch. Antero-superior diverticulum i n middle of canal.
33. Clear.
34. Slight. opposed elevations anteriorly o n inner margin. Inferior elevation
stronger.
35. Large. Clear.
36. Large. Opposed spurs between anterior and middle thirds.
37. Clear.
38. Large. Antero-superior spur.
39. Slight ant,erior superior eminence between anterior and middle thirds.
40. Clear.
41. Clear.
42. Opposed elcvations and dried remains of a connective tissue septum.
Ant,erior canal half posterior.
43. Slight superior spur between anterior and middle thirds of internal
margin.
44. Median and anterior superior spurs with slight opposed inferior ridges.
A small canal from anterior superior corner returns by a curved course to superior wall near external orifice.
45. (IV.) Antero-superior notch.
46. Two superior internal spurs equally spaced with a n anterior medial spur
opposed t o the anterior superior spur. A canal from posterior wall probably t o
posterior condyloid ca.naI.
47. An anterior c~onstriction.
48. Sniell 1 t o 1.5 mm. round notch o n antero-medial internal margin which
is set off by two spurs.
49. Antero-supcrior notch.
VARIATIONS O F T H E CANALIS HYPOGLOSSI
143
50. Median superior tubercle.
51. (5llt.) Strong horizontal septum. Smallcr inferior, larger superior
ranal.
52. Large. Clear.
53. Septum i n internal part of canal not quite t o inner orifice. Anterior
canal half the size of the posterior.
54. A complete thin, 2 mm. wide, supcrficial internal septum. Anterior
canal slightly smaller.
55. Two equally spaced superior internal tubercles. Antero-inferior spur
opposes anterior one.
56. Anteriorly constricted internal opening.
57. Clear.
58. Clear.
59. Clear
Although the number of skulls from different groups or types
was too small for a comprehensive view, the above table indicates a variation in the frequency of division among the different racial types. This was also found t o be true by Bartels
('04). Complete division shows itself in the Illinois Indians in
only 12.9 per cent of the cases, while in the California Indians
it was seen in 22.4 per cent. In the dissecting room specimens
and in the European skulls the incidence was 13.4 per cent and
11.4 per cent respectively.
Distinct differences between the right and left sides exist.
Complete division is more frequent on the left side-16.99 per
cent as against 11.84 per cent-while indicated division is more
common on the right-38.82 per cent as compared with 33.99
per cent. The canal is clear in an equal number of cases on
each side. Racial differences seem to be indicated here, too.
In the Illinois Indian skulls and in those from our dissecting
room there is practically no difference between the two sides,
but in the California Indians and in the European skulls decided
differences were found.
The canal is clear in an equal unmber of cases on both sides
This agrees very well with Weigner's ('11) who found the canal
clear in 62.1 per cent of the skulls on the left and in 59.2 per
cent on the right. I find 49.02 per cent on the left and 49.34
per cent on the right. Weigner found indicated division in 19.4
per cent on the left and in 18.5 per cent on the right-practi-
144
RALPH D. LILLIE
cally in equal numbers in 103 skulls. I find 33.99 per cent and
38.82 per cent for the left and right sides respectively. Complete division of the canal was present on the left in 18.5 per
cent and on the right in 22.3 per cent of Weigner’s specimens,
but in only 17.0 per cent on the left and in 11.8 per cent on the
right side in my series.
Frequently in cases of complete or indicated divisions the
two canals are unequal, one being about twice the size of the
other. The anterior usually is the smaller. Indications of
further division of the larger of the two canals occurs in a few
cases. The two canals are often equal in size. In the latter
case some few examples of further indicated division of the
anterior canal were seen.
From the variations in form of the hypoglossal canal in adult
skulls I am unable to decide whether there were three or four
hypoglossal canals developmentally.
I take pleasure in thanking Professor Meyer for his assistance
and suggestions.
LITERATURE C I T E D
BARTELS,P. 1904 TJbcr Rasseniinterscheide am Schadel I. Intern Monschr.
f . Anat. u. Physiol. Bd. 21.
BREMER,J. 1,. 1908 Aberrant roots and branches of the abducent and hypoglossal nerves. Jour. Comp. Neur. Psychol., vol. 18.
FRORIEP1911 Poirier e t Charpy. Tr. d’anat. hum. Tome 1, Paris.
JAHoULAY ET LUCY 1911 Poirier e t Charpy. Tr. d’anat. hum. Tome 1, Paris.
:MARTIN 1891 Die Entwickelung der neunten bis ewolften Kopfnerven bei der
Katee. Anat. Am. Bd. 6.
J. P. 1905 The development of the human body. Phila.
MCMURRICH,
POIRIERE T CHARPY 1911 Trait6 d’Anatomie humaine. Tome 1, Paris.
PRENTISS,C . W. 1910 The development of t h e hypoglossal ganglia of pig
embryos. Jour. Comp. Ncur. Psychol., vol. 20.
QCHAFER AND SYMINGTON1909 Quain’s Anat., vol. 3, p t . 2.
11th ed. London.
GRAF VON SPEE 1896 Bardeleben Handb. d. Anat. d. Menschen. Bd. 1,
Abth. 2.
WEIONER, K. 1911 o b e r dic Assimilation des Atlas und uber die Variationen
am 0 s occipitale beim Menschen. Anat. Hefte Bd. 45.